1. Field of the Invention
This invention relates to an AD (analog to digital) converter, and more particularly to an AD converter which includes an averaging resistance element.
2. Description of the Related Art
Commonly, a flash type AD converter (hereinafter referred to simply as “AD converter”) is widely known. While an AD converter of the type described includes a large number of amplifiers for amplifying reference voltages between an input analog signal and different reference voltages, if it is tried to implement a higher resolution, the offset between the amplifiers becomes serious, there is a tendency that the application ranges is limited.
Thus, an AD converter is known wherein an averaging resistance element is provided between outputs of those amplifiers to which reference voltages neighboring with each other are inputted to reduce the offset in order to solve the subject described.
For example, Japanese Patent Laid-Open No. 2005-136696 discloses such an AD converter as shown in FIG. 7. Referring to FIG. 7, the AD converter 100 shown includes a plurality of averaging resistance elements Rave provided in series between amplifiers 101-1 to 101-n of an amplifier group 101. In the AD converter 100, a plurality of, for example, nine, dummy circuits 102 are provided in series and terminated at an output terminal of the lowest-order amplifier 101-1 and an output terminal of the highest-order amplifier 101-n so that the offset between the amplifiers 101-1 and 101-n is reduced.
Further, the patent document mentioned above discloses also such a modified AD converter as shown in FIG. 8. Referring to FIG. 8, the AD converter 200 shown is configured such that the output terminals of the lowest- and highest-order amplifiers 101-1 and 101-n are terminated appropriately depending upon a relationship between the resistance value of output resistance (hereinafter referred to as “output resistance R1”) of the amplifiers and the resistance value of the averaging resistance devices Rave. Also it is disclosed that, in the AD converter 200, suitable termination can be achieved where the resistance value of the output resistance R1 of the amplifier 101 is 1.5Rave-0.5R1 when it is higher than the resistance value of the averaging resistance device Rave.